Switch from native HTML5 drag to react-arborist's built-in drag system
for internal node drag-and-drop. The HTML5Backend used by react-arborist
was intercepting dragstart events, preventing native drag from working.
- Add onMove callback and disableDrop validation to Tree component
- Sync react-arborist drag state (isDragging, willReceiveDrop) to Zustand
- Simplify use-node-move to only handle API execution
- Update use-unified-drag to only handle external file uploads
- External file drops continue to work via native HTML5 events